Rocky Mountain (56th Annual) and Cordilleran (100th Annual) Joint Meeting (May 3–5, 2004)

Paper No. 2
Presentation Time: 8:00 AM-5:00 PM


BROOKS, Justin M. and CRIDER, Juliet G., Geology, Western Washington Univ, 516 High Street, MS 9080, Bellingham, WA 98225,

Late Quaternary terraces dominate the landscape near Chelan, Washington. I have produced a new geomorphic map of the Columbia River corridor between Pateros and Entiat, central Washington. This work expands on previously published maps. These terraces indicate that at their maximum extents, both the western margin of the Okanogan Lobe of the Cordilleran Ice Sheet and a valley glacier occupying the Lake Chelan valley, terminated in this area. This reach of the Columbia River also served as a spillway for large glacial outburst floods (jökulhlaups) during the Late Quaternary.

A variety of glacial and post-glacial terrace and terrace-like features, including the enigmatic “Great Terrace,” occur throughout the area. Mapped units include: ice-marginal lacustrine and fluvial terraces, outwash terraces, fill-cut terraces, and glacial-outburst gravel bars. The maximum elevations of glacial features (erratics, kames etc.) and trimlines lie between 400 and 600 m above the modern Columbia River. The Great Terrace, which is an extensive, broad, low-gradient bench (less than 0.6 m/km) marked by muted kettles, rises roughly 175 m above the modern Columbia. Along gradient it changes character from apparently ice-contact related to outwash dominated south of Chelan. Low relief fill-cut terraces are inset into the margins of the Great Terrace. The lowest terraces (less than 100 m above the modern Columbia River) are glacial outburst gravel bars and the modern flood plain.

This new map provides potential constraints on the relative ages of the Okanogan Lobe and Lake Chelan glaciers. Mapped terrace treads will also provide baselines to evaluate deformation related to the 1872 Washington State Earthquake and the Chelan Seismic Zone.